Extensive decentralized hydrogen export from the Atlantis Massif

Hydrogen is an important energy source for subsurface microbial communities, but its availability beyond the flow focused through hydrothermal chimneys is largely unknown. We report the widespread export of H2 across the Atlantis Massif oceanic core complex (30°N, Mid-Atlantic Ridge; up to 44 nM), which is distinct from the circulation system feeding the Lost City Hydrothermal Field (LCHF) on the massif's southern wall. Methane (CH4) abundances are generally low to undetectable (<3 nM) in fluids that are not derived from the LCHF. Reducing fluids exit the seafloor over a wide geographical area and depth range, including the summit of the massif and along steep areas of mass wasting east of the field. The depth of the fluids in the water column and their H2/CH4 ratios indicate that some are sourced separately from the LCHF. We argue that extensive H2 export is the natural consequence of fluid flow pathways strongly influenced by tectonic features and the volume and density changes that occur when ultramafic rocks react to form serpentinites, producing H2 as a by-product. Furthermore, the circulation of H2-rich fluids through uplifted mantle rocks at moderate temperatures provides geographically expansive and stable environmental conditions for the early evolution of biochemical pathways. These results provide insight into the spatial extent of H2- and CH4-bearing fluids associated with serpentinization, independent of the focused flow emanating from the LCHF.